Method For Transmitting Channel Quality Information Based On Differential Scheme

ABSTRACT

A method for transmitting channel quality information based on a differential scheme is disclosed. When channel quality information of a predetermined number of sub-bands selected by a receiver in a frequency selective channel is transmitted, total average channel information is transmitted. Channel information of the selected sub-bands is transmitted as sub-band differential information associated with average channel information. In this case, the sub-band differential information may be denoted by a specific value contained in a differential-value range including only positive (+) values. If at least two channel quality information is transmitted by a MIMO system, channel quality information of one channel is transmitted, then channel quality information of the other channel is transmitted as spatial differential information. In this case, the spatial differential information is denoted by a specific value contained in a differential-value range asymmetrical on the basis of “0”.

This application is a continuation of and claims the benefit of, U.S.application Ser. No. 12/448,908, filed Jul. 15, 2009, which is aNational Stage Entry of International Patent Application No.PCT/KR2008/000674, filed on Feb. 4, 2008, and claims the benefit ofKorean Patent Application No. 10-2007-0092013, filed Sep. 11, 2007,Korean Patent Application No. 10-2008-0009041, filed on Jan. 29, 2008,U.S. Provisional Patent Application No. 60/888,298, filed Feb. 5, 2007,U.S. Provisional Patent Application No. 60/894,870, filed Mar. 14, 2007,and U.S. Provisional Patent Application No. 61/018,663, filed Jan. 2,2008, and is related to U.S. patent application Ser. No. 12/588,499,filed Oct. 16, 2009, the contents of each of the above mentionedapplications are hereby incorporated by reference herein for allpurposes as if set forth in their entireties.

TECHNICAL FIELD

The present invention relates to a method for transmitting channelquality information using a mobile communication system, and moreparticularly to a method for effectively reducing/transmitting theamount of feedback for channel quality information on the basis of adifferential scheme in a frequency selective channel, and a method foreffectively transmitting channel quality information on the basis of adifferential scheme in a Multiple Input Multiple Output (MIMO) system.

BACKGROUND ART

A channel quality indicator (CQI) for indicating channel qualityinformation to be described later in the present invention willhereinafter be described.

In order to implement an effective communication system, a receiverneeds to inform a transmitter of feedback channel information.Generally, the receiver transmits downlink channel information throughan uplink, and transmits uplink channel information through a downlink.This above-mentioned channel information is called a channel qualityindicator (CQI).

The above-mentioned CQI can be generated in various ways. For example,channel state information is quantized without any change, so that theCQI can be transmitted using the quantized channel state information. ASignal to Interference and Noise Ratio (SINR) is calculated, and the CQIis transmitted according to the calculated SINR. And, the CQI may informactual application status information of a channel in the same manner asin a Modulation Coding Scheme (MCS).

There are many cases for generating the CQI on the basis of the MCS inthe above-mentioned CQI generation methods, so that their detaileddescription will hereinafter be described.

For example, the CQI can be generated for an HSDPA transmission schemebased on the 3rd Generation Partnership Project (3GPP). In this way, ifthe CQI is generated on the basis of the MCS, the MCS includes amodulation scheme, a coding scheme, and an associated coding rate, etc.Therefore, if the modulation scheme and the coding scheme are changed,the CQI must also be changed, so that a minimum number of CQI requiredfor a codeword unit is at least 1.

If the MIMO scheme is applied to a system, the number of required CQIsis changed. In other words, the MIMO system generates multiple channels(i.e., a multi-channel) using multiple antennas (i.e., a multi-antenna),so that a plurality of codewords can be used for the MIMO system. As aresult, the MIMO system must also use a plurality of CQIs. In this way,if many CQIs are used for the MIMO system, an amount of controlinformation required for the CQIs proportionally increases.

FIG. 1 is a conceptual diagram illustrating a method forgenerating/transmitting the CQI.

Referring to FIG. 1, the user equipment (UE) 100 measures a downlinkchannel quality, selects a CQI value representing a CQI index on thebasis of the measured downlink channel quality, and reports the selectedCQI value over an uplink control channel to the Node-B 200. The Node-B200 performs downlink scheduling (e.g., UE selection, resourceallocation, etc.) according to the reported CQI value.

In this case, the CQI value may represent a CQI index based on a Signalto Interference and Noise Ratio (SINK), a Carrier to Interference andNoise Ratio (CINR), a Bit Error Rate (BER), a Frame Error Rate (FER), orassociated calculation value configured in the form of transmittabledata. In the case of the MIMO system, Rank Information (RI) or PrecodingMatrix Information (PMI) may be added to channel state information.

In the meantime, a mobile communication system employs a link adaptationto maximally use a channel capacity, and adjusts a Modulation and CodingSet (MCS) and a Transmission Power (TP) according to a given channel. Inorder to perform the above-mentioned link adaptation at the Node-B, theuser equipment (UE) must feed back channel quality information to theNode-B.

If a frequency band used by the system has a bandwidth wider than acoherent bandwidth, a channel status is abruptly changed within anentire bandwidth.

Specifically, a multi-carrier system such as an Orthogonal FrequencyDivision Multiplexing (OFDM) system has a plurality of sub-carrierswithin a given bandwidth. The multi-carrier system transmits a modulatedsymbol via each sub-carrier, so that its optimum transmission is thatthe each subcarrier channel is considered when transmitting data.

Therefore, an amount of feedback channel information abruptly increasesin the multi-carrier system including several sub-carriers, so that thedemand of developing an improved method for reducing overhead of controlsignals is rapidly increased.

DISCLOSURE Technical Problem

Accordingly, the present invention is directed to a method fortransmitting channel quality information on the basis of a differentialscheme that substantially obviates one or more problems due tolimitations and disadvantages of the related art.

An object of the present invention is to provide a method for reducingoverhead generated when channel quality information is transmitted overa frequency selective channel.

Another object of the present invention is to provide a differential CQIreporting scheme to indicate a differential CQI using less amount ofcontrol information, a method for defining the range of a differentialvalue indicating the differential CQI to represent more correct channelstate information, and a quantization method for indicating thedifferential CQI.

Another object of the present invention is to provide a method forreducing overhead over the frequency selective channel, and/or a methodfor reducing overhead generated when channel quality information istransmitted via a MIMO system.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

Technical Solution

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, amethod for a user equipment (UE) to transmit a channel quality indicator(CQI) to a base station is provided. The method comprises: selecting apreferred set of M subbands among a set of N subbands, wherein the setof N subbands spans an entire downlink system bandwidth; reporting thepositions of the M selected subbands using a combinatorial indexselected from among {0, 1, . . . , NCM−1}; reporting one CQI valuerepresenting wideband CQI index for the set of N subbands; and reportingone differential CQI value reflecting the transmission of the basestation only over the M selected subbands among a predetermined numberof differential CQI values, wherein each of the differential CQI valuesis defined based on an offset level corresponding to:

a CQI index for the M selected subbands—the wideband CQI index, and

wherein the differential CQI values represent more of a first case inwhich the offset level has a positive value than a second case in whichthe offset level has a negative value.

Preferably, when the UE selects the preferred set of M subbands, the UEreports the positions of subbands using the combinatorial indexrepresenting the M selected subbands.

Preferably, each of the subbands comprises X frequency domain units, theentire downlink system bandwidth comprises Y frequency domain units, andthe number of subbands spanning the entire downlink system bandwidth (N)corresponds to an integer number not less than Y/X.

Preferably, each of the subbands comprises a plurality of subcarriers.

Preferably, the differential CQI value is represented as a 2-bit value.

Preferably, the positions of the M selected subbands, the CQI valuerepresenting wideband CQI index and the differential CQI value arereported via a Physical Uplink Shared Channel (PUSCH).

To achieve the above objects and other advantages and in accordance withthe purpose of the invention, as embodied and broadly described herein,a user equipment (UE) for transmitting a channel quality indicator (CQI)to a base station is provided. The UE comprises: a receiver configuredto receive signals from the base station; a processor configured toselect a preferred set of M subbands among a set of N subbands, whereinthe set of N subbands spans an entire downlink system bandwidth; and atransmitter configured to report the positions of the M selectedsubbands using a combinatorial index selected from among {0, 1, . . .NCM−1}, one CQI value representing wideband CQI index for the set of Nsubbands, and one differential CQI value reflecting the transmission ofthe base station only over the M selected subbands among a predeterminednumber of differential CQI values, wherein each of the differential CQIvalues is defined based on an offset level corresponding to:

a CQI index for the M selected subbands—the wideband CQI index, and

wherein the differential CQI values represent more of a first case inwhich the offset level has a positive value than a second case in whichthe offset level has a negative value.

Preferably, when the processor selects the preferred set of M subbands,the transmitter reports the positions of subbands using thecombinatorial index representing the M selected subbands.

Preferably, each of the subbands comprises X frequency domain units, theentire downlink system bandwidth comprises Y frequency domain units, and

wherein the number of subbands spanning the entire downlink systembandwidth (N) corresponds to an integer number not less than Y/X.

Preferably, each of the subbands comprises a plurality of subcarriers.

Preferably, the differential CQI value is represented as a 2-bit value.

Preferably, the positions of the M selected subbands, the CQI valuerepresenting wideband CQI index and the differential CQI value arereported via a Physical Uplink Shared Channel (PUSCH).

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Advantageous Effects

According to the above-mentioned embodiments of the present invention,if the CQI is represented by the differential CQI scheme over thefrequency selective channel, and a reference value based on thedifferential scheme is a CQI average value of all corresponding bands, adifferential value for indicating the differential CQI is effectivelyestablished, so that correct channel quality information can betransmitted with less number of bits.

In more detail, the frequency selective channel transmits the CQIaccording to a sub-band differential CQI scheme, so that the CQIassociated with a plurality of sub-bands can be effectively transmitted.The MIMO system transmits the CQI associated with the plurality ofsub-bands according to the spatial differential CQI scheme, so that itcan additionally reduce an amount of overhead.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 is a conceptual diagram illustrating a CQI generation andtransmission scheme;

FIG. 2 is a conceptual diagram illustrating a method for generating aCQI by selectively establishing a CQI sub-band in a frequency domain;

FIG. 3 is a conceptual diagram illustrating a general MIMO system;

FIG. 4 is a graph illustrating a probability distribution difference ofa differential CQI value according to a reception (Rx) method of areceiver;

FIG. 5 is a graph illustrating a method for quantizing a differentialchannel information value according to an embodiment of the presentinvention;

FIG. 6 is another graph illustrating a method for quantizing adifferential channel information value according to an embodiment of thepresent invention;

FIG. 7 exemplarily shows a channel-value distribution of each codewordwhen two codewords are received over several unit frequency bands;

FIG. 8 is a simulation result illustrating a distribution ofdifferential channel information in each unit frequency band when twocodewords are transmitted; and

FIG. 9 is a simulation result illustrating a method for comparing aconventional channel information transmission method with an inventivechannel information transmission method.

BEST MODE

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

Prior to describing the present invention, it should be noted that mostterms disclosed in the present invention correspond to general termswell known in the art, but some terms have been selected by theapplicant as necessary and will hereinafter be disclosed in thefollowing description of the present invention. Therefore, it ispreferable that the terms defined by the applicant be understood on thebasis of their meanings in the present invention. For example, thefollowing description will disclose detailed examples of theabove-mentioned 3GPP LTE (3rd Generation Partnership Project Long TermEvolution) system, the scope or spirit of the present invention is notlimited to only the 3GPP LTE system, and can also be applied to othercommunication systems which may require a feedback of downlink channelquality information.

For the convenience of description and better understanding of thepresent invention, the following detailed description will disclose avariety of embodiments and modifications of the present invention. Insome cases, in order to prevent ambiguous concepts of the presentinvention from occurring, conventional devices or apparatuses well knownto those skilled in the art will be omitted and denoted in the form of ablock diagram on the basis of the important functions of the presentinvention.

Firstly, the present invention may change a transmission unit of channelinformation to another unit. For example, according to the OFDM scheme,the present invention may combine transmission (Tx) channel informationfor several sub-carriers into a single sub-carrier group, and transmitsthe combined channel information in units of the corresponding group. Inother words, if the OFDM scheme based on 2048 sub-carriers collects 12sub-carriers to form a single sub-carrier group, a total of 171sub-carrier groups are formed, so that the number of actual Tx channelinformation units is reduced from 2048 to 171.

According to the following description, if individual frequency bandsare distinguished by individual sub-carriers, respectively, in the samemanner as in the OFDM scheme, the present invention combines one or moresub-carriers into a single group, and reports a CQI for each sub-carriergroup unit as a basic unit. In this case, the basic unit is called a CQIsub-carrier group or CQI sub-band.

In the meantime, if the individual frequency bands are not distinguishedfrom each other as in the individual sub-carriers, an overall frequencyband is divided into several frequency bands, and the CQI is generatedon the basis of the divided frequency bands. Each divided frequency bandfor the CQI generation is called a CQI sub-band. This CQI sub-band willhereinafter be referred to as a “sub-band”.

Next, the present invention may generate the CQI by compressing channelinformation. For example, the OFDM system compresses channel informationfor each sub-carrier using a specific compression scheme, and transmitsthe compressed channel information. For example, the present inventionmay consider a variety of compression methods such as a Discrete CosineTransform (DCT).

Also, the present invention may select a corresponding frequency bandfor generating channel information, and may generate the CQI using theselected frequency band. The OFDM system may selectively transmit best Msub-carriers or best M sub-carrier groups, instead of transmittingchannel information to each of all sub-carriers. For example, a Best-Mscheme may be used as a selective transmission scheme.

When the CQI is transmitted over the selected frequency band, an actualtransmission part can be generally divided into two parts, i.e., aCQI-value part and a CQI subband-index part.

FIG. 2 is a conceptual diagram illustrating a method for generating aCQI by selectively establishing a CQI sub-band in a frequency domain.

In a graph shown in an upper part of FIG. 2, a horizontal axis is afrequency axis, and a vertical axis is a CQI value of each frequencydomain. In the graph shown in the upper part of FIG. 2, the horizontalaxis is divided into grouping sub-band units of several sub-carriers,and indexes are assigned to individual sub-bands, respectively.

A frequency-domain selective CQI method (i.e., a frequency selective CQImethod) generally includes the following three steps. In the first step,the frequency selective CQI method selects a CQI sub-band in which theCQI will be generated. In the second step, the frequency selective CQImethod manipulates CQI values of the selected frequency bands, andgenerates/transmits the CQI values. In the third step, the frequencyselective CQI method transmits indexes of the selected frequency bands(i.e., CQI sub-bands).

FIG. 2 shows the Best-M scheme and the Threshold-based scheme asexamples of the method for selecting the CQI sub-band in theabove-mentioned first step.

The Best-M scheme is adapted to select M CQI sub-bands having a goodchannel status. FIG. 2 shows an exemplary method for selecting CQIsub-bands of Nos. 5, 6, and 9 indexes having a good channel status usingthe Best-3 scheme. The threshold-based scheme selects the CQI sub-bandhaving a channel status value higher than a predetermined thresholdvalue (T). As can be seen from FIG. 2, the threshold-based schemeselects the Nos. 5 and 6 CQI sub-bands having a channel status valuehigher than the threshold value (T).

In the meantime, FIG. 2 shows exemplary methods forgenerating/transmitting CQI values in the second step, i.e., anindividual transmission scheme and an average transmission scheme.

The individual transmission scheme transmits all CQI values of theselected CQI sub-bands of the above-mentioned first step. Therefore,according to the individual transmission scheme, the higher the numberof selected CQI sub-bands, the higher the number of CQIs to betransmitted.

In the meantime, the average transmission scheme transmits an averagevalue of CQI values of the selected CQI sub-bands. Therefore, theaverage transmission scheme has a single CQI value to be transmitted,irrespective of the number of selected CQI sub-bands. If an averagevalue of several CQI sub-bands is transmitted, accuracy is deteriorated.In this case, the average value of the CQI values may be calculated byan arithmetic average scheme or a channel capacity average scheme.

As shown in FIG. 2, the CQI generation/transmission method in the secondstep is associated with the first step in which Nos. 5, 6, and 9 CQIsub-bands are selected by the Best-3 scheme. In other words, if theindividual transmission scheme is used in the second step, CQI values 7,6, and 5 of Nos. 5, 6, and 9 sub-bands are generated and transmittedrespectively. If the average transmission scheme is used in the secondstep, CQI values of Nos. 5, 6, and 9 sub-bands arithmetically average to“6”.

FIG. 2 shows exemplary methods for transmitting the index of the CQIsub-band in the third step, for example, a bitmap index scheme and ageneral combinatorial index scheme.

The bitmap index scheme assigns a single bit to each of all CQIsub-bands. If a corresponding CQI sub-band is used, “1” is assigned tothe corresponding CQI sub-band. Otherwise, if a corresponding CQIsub-band is not used, “0” is assigned to the corresponding CQI sub-band.The bitmap index scheme indicates which one of CQI sub-bands will beused. The bitmap index scheme requires several bits as many as total CQIsub-bands. However, the bitmap index scheme may indicate which one ofCQI sub-bands will be used using a fixed number of bits withoutconsidering how many CQI sub-bands will be used.

In the meantime, the combinatorial index scheme determines how many CQIsub-bands will be used, maps individual cases of combinationscorresponding to the number of used CQI sub-bands from among a total ofCQI sub-bands to individual indexes, and indicates the mapping result.In more detail, if a total of N CQI sub-bands exist, and M CQI sub-bandindexes from among N CQI sub-band indexes are used to generate the CQI,a total number of available combinations can be represented by thefollowing equation 1:

$\begin{matrix}{{{}_{}^{}{}_{}^{}} = \frac{N!}{{M!}{\left( {N - M} \right)!}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

The number of bits indicating the number of cases shown in Equation 1can be represented by the following equation 2:

$\begin{matrix}{\left\lceil {\log_{2}\left( {{}_{}^{}{}_{}^{}} \right)} \right\rceil = \left\lceil {\log_{2}\left( \frac{N!}{{M!}{\left( {N - M} \right)!}} \right)} \right\rceil} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

FIG. 2 shows the method for selecting three CQI sub-bands from among atotal of 11 CQI sub-bands. In FIG. 2, the number of available cases is₁₁C₃=165. The number of bits to indicate the above-mentioned 165 casesis 8 (i.e., 8 bits), as denoted by 2⁷≦₁₁C₃≦2⁸.

The CQI transmission method based on the differential scheme isclassified into a first CQI transmission method according to a firstembodiment and a second CQI transmission method according to a secondembodiment. The first CQI transmission method applies theabove-mentioned CQI transmission method to transmit a CQI of eachsub-band over the frequency selective channel. The second CQItransmission method applies the above-mentioned CQI transmission methodto spatially transmit a CQI of at least two channels by the MIMO system.

First Embodiment Sub-Band Differential CQI Transmission Method

According to the following first embodiment, the present inventiongenerates/transmits the CQI using the Best-M scheme from among theabove-mentioned CQI generation/transmission schemes, converts a CQIvalue to be transmitted to M sub-bands into differential informationassociated with a total-band CQI value, and reduces an amount ofoverhead.

In the case of indicating CQI values of individual bands using thefrequency-band selective CQI generation method, a differential schemecan be applied to indicate the above-mentioned CQI values. In moredetail, an average value of CQI values of a total frequency band actingas a reference is calculated, and CQI values of a frequency bandselected by a receiver can be represented on the basis of the calculatedaverage value. In other words, an average value of the CQI values ofeach selected frequency band is calculated, and the average value iscompared with a reference value, so that the resultant CQI values can berepresented by a difference between the reference value and each averagevalue.

If a correlation between the average value and each CQI value exists inthe case of using the above-mentioned differential scheme, the presentinvention can effectively represent the CQI value with less number ofdifferential values. The following first embodiment of the presentinvention will disclose a method for indicating differential informationwith less number of bits exploiting a correlation between a CQI value ofa selected sub-band and an average CQI of all bands.

For the convenience of description, it is assumed that the firstembodiment indicates the frequency selective CQI in the form ofdifference values based on a total average according to the differentialscheme. In order to implement the frequency selective CQI method, thereceiver is designed to basically select only CQI sub-bands having arelatively good channel status. Therefore, there is every probabilitythat each CQI value of the selected CQI sub-bands is higher than a totalaverage CQI value. Needless to say, strictly speaking, as the number ofthe selected CQI sub-bands is gradually less than a total number of CQIsub-bands, the probability that each CQI value of the selected CQIsub-bands is higher than a total average CQI value gradually increases.Generally, most frequency selective CQI methods select only a smallnumber of good bands from among a total broad band, so that it can beconsidered that each of the CQI values of the selected CQI sub-bands behigher than the total average CQI value.

For example, a method for transmitting CQI values of the selectedsub-bands 5, 6, and 9 of the Best-3 scheme shown in FIG. 2 willhereinafter be described in detail.

Referring to FIG. 2, an average CQI value of a total frequency bandserving as a CQI transmission reference is 3, as denoted by“3=(0+1+2+1+4+7+6+3+4+5+0)/11”. Therefore, the CQI values 7, 6, and 5 ofthe sub-bands 5, 6, and 9 selected by the receiver according to theBest-3 scheme are converted into differential values (i.e., 4, 3, and 2)associated with the value of reference value 3, respectively. It can berecognized that the converted differential values 4, 3, and 2 are allpositive (+).

Therefore, provided that the frequency selective CQI scheme has used thetotal average CQI value as a reference value, it can consider a specificdirectivity (or orientation) when establishing the range of a differencevalue indicating differential information.

In more detail, CQI values of the CQI sub-bands selected by the receiverare generally higher than a reference value equal to a total average CQIvalue, so that a differential value may be generally equal to or higherthan “0”. Therefore, in the case of establishing the differential-valuerange for indicating the differential value, a method for consideringonly the remaining values other than negative(−) values may beconsidered to be effective. Therefore, the first embodiment of thepresent invention proposes a method for establishing thedifferential-value range using the above-mentioned description,configuring each CQI of the selected sub-bands of the receiver in theform of a specific value within such a differential value range, andtransmitting the resultant CQI configured in the form of the specificvalue.

For example, if 3 bits are assigned for the differential scheme, 8values can be established, so that the setup range of the differentialvalue is set to [−3 −2 −1 0 1 2 3 4] in consideration of all positive(+)and negative(−) values. However, if only specific values higher than “0”are considered, the setup range of the differential value is set to [0 12 3 4 5 6 7], so that a wider area can be denoted by given bits.

Therefore, provided that the differential CQI scheme is used as thefrequency selective CQI transmission scheme, and a reference value isused as a CQI average value of a total CQI reporting band, the firstembodiment of the present invention provides a method for consideringonly the remaining areas other than negative (−) areas in the case ofestablishing a difference value indicating a differential CQI.

In the meantime, the above-mentioned method can be applied to not onlythe method for transmitting CQIs of the selected frequency bands,respectively, but also the method for transmitting an average value ofthe CQI values of the selected frequency bands. In other words, adifference value between a desired average value of the CQI values ofthe selected frequency bands and another average value of CQI values ofall corresponding frequency bands is calculated, so that the desiredaverage value may also be represented by the above-mentioneddifferential value.

For example, in addition to the above-mentioned Best-3 scheme (See FIG.2) in which CQI values 7, 6, and 5 of the selected sub-bands 5, 6, and 9selected by the receiver have been converted into differential values(i.e., 4, 3, and 2) associated with the reference value of 3 equal tothe total average CQI value, the above-mentioned first embodiment mayalso transmit an average value “6” of the CQI values of the selectedsub-bands 5, 6, and 9, or may also transmit a difference value “3”between the average value “6” of selected sub-bands and the CQI averagevalue “3” of all bands.

According to the above-mentioned first embodiment, negative(−) valueswill not be used when a differential-value range for indicating thedifferential CQI is established, so that only the remaining values otherthan the negative values are used when the differential value range isestablished. However, as previously stated above, if a differencebetween a corresponding bandwidth and a selected bandwidth is verysmall, there is a low probability that the differential CQI value has apositive(+) value. Therefore, in order to more stably establish therange of a difference value, only positive(+) values are mainlyconsidered, but some negative(−) values may also be considered asnecessary. Namely, when the difference value range for the differentialCQI is established, the range of positive and negative values does nothave a symmetrical shape on the basis of “0”, and has an asymmetricallyskewed shape on the basis of “0”. In more detail, in the case of theasymmetrical shape, the allocation range of positive(+) and negative(−)parts leans to the positive(+) part on the basis of “0”, so that theasymmetrical range inclined to the positive part is formed. In this way,the first embodiment provides the above-mentioned method for forming theasymmetrical range inclined to the positive part on the basis of “0”, sothat it may effectively consider the formed asymmetrical range.

In the meantime, the above-mentioned method can also be applied to notonly the individual transmission scheme for transmitting each CQI of theselected frequency band, but also the average transmission scheme foraveraging CQI values of the selected frequency band. In other words, theabove-mentioned method can also be applied to the method forrepresenting an average value of CQI values of the selected frequencyband using a difference value between the CQI values of the selectedfrequency band and an average value of CQI values of all correspondingfrequency bands.

Second Embodiment Spatial Differential CQI Transmission Method

According to the following second embodiment, the present inventionsimultaneously or independently configures CQIs of several channels inthe form of a differential value on the basis of a specific CQI of anyone of the channels, and transmits the differential-formatted CQIs, sothat it reduces an amount of overhead. A detailed description of thesecond embodiment will hereinafter be described with reference to theMIMO system.

A Multi-Input Multi-Output (MIMO) technology will hereinafter bedescribed in detail.

In brief, the MIMO technology is an abbreviation of the Multi-InputMulti-Output technology. The MIMO technology uses multiple transmission(Tx) antennas and multiple reception (Rx) antennas to improve theefficiency of Tx/Rx data, whereas a conventional art has generally useda single transmission (Tx) antenna and a single reception (Rx) antenna.In other words, the MIMO technology allows a transmitter or receiver ofa wireless communication system to use multiple antennas (hereinafterreferred to as a multi-antenna), so that the capacity or performance canbe improved. For the convenience of description, the term “MIMO” canalso be considered to be a multi-antenna technology.

In more detail, the MIMO technology is not dependent on a single antennapath to receive a single total message, collect a plurality of datapieces received via several antennas, and complete total data. As aresult, the MIMO technology can increase a data transfer rate within aspecific range, or can increase a system range at a specific datatransfer rate. In other words, the MIMO technology is thenext-generation mobile communication technology capable of being appliedto mobile communication terminals or repeaters.

The next-generation mobile communication technology requires a datatransfer rate higher than that of a conventional mobile communicationtechnology, so that it is expected that the effective MIMO technology isrequisite for the next-generation mobile communication technology. Underthis situation, the MIMO communication technology is the next-generationmobile communication technology capable of being applied to mobilecommunication terminals or repeaters, and can extend the range of a datacommunication range, so that it can overcome the limited amount oftransfer data of other mobile communication systems due to a variety oflimited situations.

In the meantime, the MIMO technology, which uses multiple antennas atall transmitters/receivers, from among a variety of technologies capableof improving the transfer efficiency of data can greatly increase anamount of communication capacity and Tx/Rx performances withoutallocating additional frequencies or increasing additional power. Due tothese technical advantages, most companies or developers are intensivelypaying attention to this MIMO technology.

FIG. 3 is a conceptual diagram illustrating a general MIMO system.

Referring to FIG. 3, if the number of transmission (Tx) antennasincreases to a predetermined number, and at the same time the number ofreception (Rx) antennas increases to a predetermined number, atheoretical channel transmission capacity of the MIMO system increasesin proportion to the number of antennas, differently from theabove-mentioned case in which only a transmitter or receiver usesseveral antennas, so that a frequency efficiency can greatly increase.

After the above-mentioned theoretical capacity increase of the MIMOsystem has been demonstrated in the mid-1990s, many developers areconducting intensive research into a variety of technologies which cansubstantially increase a data transfer rate using the theoreticalcapacity increase. Some of them have been reflected in a variety ofwireless communication standards, for example, a third-generation mobilecommunication or a next-generation wireless LAN, etc.

A variety of MIMO-associated technologies have been intensivelyresearched by many companies or developers, for example, research intoan information theory associated with a MIMO communication capacitycalculation under various channel environments or multiple accessenvironments, research into a wireless channel measurement and modelingof the MIMO system, and research into a space-time signal processingtechnology.

The above-mentioned MIMO technology can be classified into a spatialdiversity scheme and a spatial multiplexing scheme. The spatialdiversity scheme increases transmission reliability using symbolspassing on various channel paths. The spatial multiplexing schemesimultaneously transmits a plurality of data symbols via a plurality ofTx antennas, so that it increases a transfer rate of data. In addition,the hybrid scheme of the spatial diversity scheme and the spatialmultiplexing scheme has also been recently developed to properly acquireunique advantages of the two schemes.

Detailed descriptions of the spatial diversity scheme, the spatialmultiplexing scheme, and the hybrid scheme thereof will hereinafter bedescribed in detail.

Firstly, the spatial diversity scheme will hereinafter be described. Thespatial diversity scheme is classified into a space-time block codescheme and a space-time trellis code scheme which can simultaneously usea diversity gain and a coding gain. Generally, a bit error ratio (BER)improvement performance and a code-generation degree of freedom of thespace-time trellis code scheme are superior to those of the space-timeblock code scheme, whereas the receiver complexity of the space-timeblock code scheme is superior to that of the space-time Trellis codescheme.

The above-mentioned spatial diversity gain corresponds to the product ormultiplication of the number of Tx antennas and the number of Rxantennas.

In the meantime, if the space-time coding scheme is considered in afrequency domain instead of the time domain, it may also be consideredto be the space-frequency coding scheme. As a result, the same scheme isapplied to not only the frequency domain but also the time domainwithout any changes.

Secondly, the spatial multiplexing scheme will hereinafter be described.The spatial multiplexing scheme is adapted to transmit different datasequence via individual Tx antennas. In this case, a receiver mayunavoidably generate mutual interference between data piecessimultaneously transmitted from transmitter antennas. The receiverremoves this mutual interference from the received data using a propersignal processing technique, so that it can receive the resultant datahaving no interference. In order to remove noise or interference fromthe received data, a maximum likelihood receiver, a ZF receiver, a MMSEreceiver, a D-BLAST, or a V-BLAST may be used. Specifically, if atransmitter can recognize channel information, a Singular ValueDecomposition (SVD) scheme may be used to remove the noise orinterference.

Thirdly, the hybrid scheme of the spatial diversity scheme and thespatial multiplexing scheme will hereinafter be described. Provided thatonly a spatial diversity gain is acquired, the performance-improvementgain is gradually saturated in proportion to an increasing diversityorder. Otherwise, provided that only the spatial multiplexing gain isacquired, a transmission reliability of a wireless channel is graduallydeteriorated.

As a result, a variety of schemes capable of acquiring all theabove-mentioned two gains simultaneously while solving theabove-mentioned problems have been intensively researched by manycompanies or developers, for example, a double-STTD scheme and aspace-time BICM (STBICM) scheme.

A general communication system performs coding of transmissioninformation of a transmitter using a forward error correction code, andtransmits the coded information, so that an error experienced at achannel can be corrected by a receiver. The receiver demodulates areceived (Rx) signal, and performs decoding of forward error correctioncode on the demodulated signal, so that it recovers the transmissioninformation. By the decoding process, the Rx-signal error caused by thechannel is corrected.

Each of all forward error correction codes has a maximum-correctablelimitation in a channel error correction. In other words, if a reception(Rx) signal has an error exceeding the limitation of a correspondingforward error correction code, a receiver is unable to decode the Rxsignal into information having no error. Therefore, the receiver mustdetect the presence or absence of an error after decoding the receivedinformation. In this way, a specialized coding process for performingerror detection is required, separately from the forward errorcorrection coding process. Generally, a Cyclic Redundancy Check (CRC)code has been used as an error detection code.

The CRC method is an exemplary coding method for performing the errordetection. Generally, the transmission information is coded by the CRCmethod, and then the forward error correction code is applied to theCRC-coded information. A single unit coded by the CRC and the forwarderror correction code is generally called a codeword.

In the meantime, if several transmission information units areoverlapped and then received, the present invention can expectperformance improvement using an interference-cancellation receiver.There are many cases in the above-mentioned case in which sometransmission information is overlapped and then received, for example, acase in which the MIMO technology is used, a case in which a multi-userdetection technology is used, and a case in which a multi-codetechnology is used. A brief description of the interference-cancellationstructure will be as follows.

According to the interference-cancellation structure, after firstinformation is demodulated/decoded from a total reception signal inwhich some information is overlapped, information associated with thefirst information is removed from the total reception signal. A secondsignal is demodulated/decoded by the resultant signal without firstinformation by removing from the reception signal. A third signal isdemodulated/decoded by the resultant signal without first- andsecond-information by removing from the first reception signal. A fourthsignal or other signal after the fourth signal repeats theabove-mentioned processes, so that the fourth or other signal isdemodulated/decoded.

In order to use the above-mentioned interference cancellation method,the demodulated/decoded signal removed from the reception signal musthave no errors. If any errors occur in the demodulated/decoded signal,error propagation occurs so that a negative influence continuouslyaffects all the demodulated/decoded signals.

The above-mentioned interference cancellation technology can also beapplied to the MIMO technology. In order to use the above-mentionedinterference cancellation technology, several transmission informationpieces must be overlapped/transmitted via multiple antennas. In otherwords, if the spatial multiplexing technology is used, each transmissioninformation is detected, and at the same time the interferencecancellation technology can be used.

However, as described above, in order to minimize the error propagationcaused by the interference cancellation, it is preferable that theinterference is selectively removed after determining the presence orabsence of an error in the demodulated/decoded signal. A representativemethod for determining the presence or absence of the error in eachtransmission information is the above-mentioned cyclic redundancy check(CRC) method. A unit of distinctive information processed by the CRCcoding is called a codeword. Therefore, a more representative method forusing the interference cancellation technology is a specific case inwhich several transmission information pieces and several codewords areused.

In the meantime, the fading channel is a major cause of deterioration ofa performance of a wireless communication system. A channel gain valueis changed according to time, frequency, and space. The lower thechannel gain value, the lower the performance. A representative methodfor solving the above-mentioned fading problem is diversity. Thisdiversity uses the fact that there is a low probability that allindependent channels have low gain values at the same time. A variety ofdiversity methods can be applied to the present invention, and theabove-mentioned multi-user diversity is considered to be one of them.

If several users are present in a cell, channel gain values ofindividual users are statistically independent of each other, so thatthe probability that all the users have low gain values is very low. Ifa Node-B has sufficient transmission (Tx) power and several users arepresent in a cell, it is preferable that all channels be allocated to aspecific user having the highest channel gain value to maximize a totalchannel capacity. The multi-user diversity can be classified into threekinds of diversities, i.e., a temporal multi-user diversity, a frequencymulti-user diversity, and a spatial multi-user diversity.

The temporal multi-user diversity is adapted to allocate a channel to aspecific user having the highest gain value when a channel situationchanges with time.

The frequency multi-user diversity is adapted to allocate a sub-carrierto a specific user having the highest gain value in each frequency bandin a frequency multi-carrier system such as an Orthogonal FrequencyDivision Multiplexing (OFDM) system.

If a channel situation slowly changes with time in another system whichdoes not use the multi-carrier, the user having the highest channel gainvalue will monopolize the channel for a long period of time, and otherusers are unable to communicate with each other. In this case, in orderto use the multi-user diversity, there is a need to induce the channelto change.

Next, the spatial multi-user diversity uses different channel gainvalues of users in space domain. An implementation example of thespatial multi-user diversity is a Random BeamForming (RBF) method (alsocalled “Opportunistic Beamforming”). This RBF method performsbeamforming with a predetermined weight using multiple antennas (i.e.,multi-antenna) to induce the change of channel, and uses theabove-mentioned spatial multi-user diversity.

In the meantime, the 3GPP LTE can use a maximum of 2 codewords. In thiscase, the 3GPP LTE requires two CQIs. In order to reduce an amount oftransmission (Tx) CQI, a differential CQI or delta CQI concept has beendeveloped. In more detail, a single CQI (i.e., a first CQI) is normallytransmitted, and the other CQI (i.e., a second CQI) can transmit only adifference between the first CQI and the second CQI itself. Thedifferential CQI or delta CQI concept uses a method similar to adifferential modulation method for use in the modulation/demodulationscheme.

However, the 3GPP LTE has not prescribed a method for indicating whichrange will include a differential CQI value to perform theabove-mentioned differential CQI reporting scheme. And, if thedifferential CQI value is quantized and transmitted, the 3GPP LTE hasnot prescribed a detailed quantization method of the differential CQIvalue.

Therefore, according to the second embodiment, the present inventionprovides the method for indicating which range will include thedifferential CQI value, and a method for transmitting channelinformation. Detailed descriptions of the above-mentioned methods willhereinafter be described.

In this case, the differential CQI is differential information betweenCQI values of two channels (i.e., two codewords), and is different fromthe sub-band differential CQI indicating differential information amongCQI values of individual sub-bands according to the first embodiment.The differential information among CQI values of several channels orcodewords will hereinafter be referred to as a “spatial differentialCQI”. If there is no confusion, it is assumed that differentialinformation is spatial differential information and the differential CQIis a spatial differential CQI respectively hereinafter.

If the above-mentioned differential CQI value is quantized according tothe second embodiment, a method for quantizing the differential CQIvalue, and indicating the quantized differential CQI value, and a methodfor additionally reducing an information amount of the differential CQIvalue when a signal is received via several unit frequency bands willhereinafter be described in detail.

When the MIMO system establishes the range of a differential CQI value,the present invention can determine the above-mentioned range inconsideration of a probability distribution of the differential CQIvalue. It should be noted that the MIMO system may have differentprobability distributions of the differential CQI value according toreception (Rx) schemes of a receiver, so that the present inventionprovides a method for establishing the indication range of thedifferential CQI value. For this purpose, a signal receiving methodusing a receiver of the MIMO system will hereinafter be described.

Generally, the maximum likelihood (ML) scheme may be considered to be anoptimum Rx method of the MIMO system. However, according to usages ofthe MIMO system, since a transmission (Tx) signal is spatially extended,the number of all cases for the ML scheme exponentially increases, sothat the ML scheme actually applied to the system causes a seriousproblem in complexity.

Two quasi-optimum methods may be considered, i.e., a first method actingas a Minimum Mean Square Error (MMSE) scheme and a second method forapplying the Successive Interference Cancellation (SIC) to the MMSEscheme. If the MMSE-based receiver instead of the ML-based receiver isused to detect space-time MIMO symbols, the MMSE-based receiver canacquire a higher advantage in complexity.

However, if only the MMSE scheme is used, a performance is less thanthat of the ML scheme. In order to reduce the performance deterioration,the above-mentioned second method for combining the MMSE and the SIC maybe considered. Firstly, this combination method of the MMSE and the SICremoves detected signals according to the interference cancellationscheme, increases the SINR of the next detection signal, and implementsa performance improvement.

For the convenience of description and better understanding of thepresent invention, the above-mentioned second method for applying theSIC to the MMSE will hereinafter be referred to as a “MMSE+SIC” scheme.If there is no confusion, the MMSE+SIC scheme may also be denoted byonly the “SIC” scheme as necessary hereinafter.

The CQI probability distribution may be changed according to adifference of the signal receiving schemes of the receiver. In order tomore quantitatively recognize the CQI difference, the followingsimulation is needed. In more detail, if the MIMO system including 4 Txantennas and 4 Rx antennas transmits two codewords under a TU channelenvironment at a moving speed of 30 km/h, the comparison result betweenCQIs of codewords according to different Rx schemes is as follows. Inthis case, it is assumed that the CQI is quantized at intervals of 1 dB.

Basically, it is assumed that the SIC scheme detects a second codewordafter detecting a first codeword.

In the meantime, in order to more correctly recognize the CQI differencebetween two Rx schemes, a CQI of a first codeword is deducted from theCQI of a second codeword, so that a difference CQI_(Delta) between thetwo CQIs can be represented by the following equation 3:

CQI_(Delta)=CQI_(Codeword2)−CQI_(Codeword1)  [Equation 3]

Differently from Equation 3, some systems may represent the spatialdifferential CQI using a value acquired when the CQI value of the secondcodeword is subtracted from another CQI value of the first codeword. Inthis case, the above-mentioned target value CQI_(Delta) has a codeopposite to that of another value CQI_(Delta), however, the target valueCQI_(Delta) can be applied to the following description in either thesame manner as in the following description or another mannersymmetrical to the following description.

FIG. 4 is a graph illustrating a probability distribution difference ofa differential CQI value according to a reception (Rx) method of areceiver.

In more detail, FIG. 4( a) shows probability distribution values of theCQI_(Delta) value under various SINR conditions when the MMSE-basedreceiver is used, and FIG. 4( b) shows probability distribution valuesof the CQI_(Delta) value under various SINR conditions when the MMSE+SICreceiver is used.

As can be seen from the probability distribution of the CQI_(Delta)value in FIG. 4, the following facts can be recognized. If theMMSE-based receiver is used, the CQI difference CQI_(Delta) between twocodewords is symmetrically distributed on the basis of “0”. If theMMSE+SIC receiver is used, the difference CQI_(Delta) between twocodewords inclines to positive(+) values on the basis of “0”.

The above-mentioned facts are theoretically reasonable. If the SIC-basedreceiver detects a second codeword, interference is removed from a firstcodeword, so that the probability of increasing the CQI of the secondcodeword is very high. In other words, according to the SIC scheme, thecloser the detection process is to the end time, the higher theprobability of improving the CQI. Therefore, the probability ofassigning a value higher than “0” to the CQI_(Delta) value is very high.

Generally, a first CQI for using in the spatial differential CQI schemetransmits all information for indicating a channel quality, and the nextCQI from the first CQI can transmit only the CQI_(Delta) valueindicating a difference between the first CQI and the aforementionednext CQI itself. In order to reduce a Tx amount of the CQI_(Delta) data,the spatial differential CQI scheme indicates the CQI_(Delta) Tx datausing a small number of bits less than those of an original CQI.Therefore, in order to effectively use the CQI_(Delta) value using thesame bits, the range denoted by the CQI_(Delta) value must be properlyestablished.

For example, as shown in FIG. 4( a), if an original value of a secondCQI is transmitted without any change on the condition the CQI_(Delta)value is not used, 21 steps (i.e., 21=10+1+10) corresponding to theinterval [−10, 10] must be represented, so that 4.39 bits (i.e.,log₂(21)) are needed. Otherwise, as shown in FIG. 4( a), if theCQI_(Delta) value is transmitted, only 13 steps (i.e., 13=6+1+6)corresponding to the interval [−6, 6] must be represented, so that 3.70bit (i.e., log₂(13)) are needed. As a result, if the CQI_(Delta) valueis used, the number of required Tx bits is reduced by about 0.69 bit.

For another example, as shown in FIG. 4( b), it is preferable that theCQI_(Delta) range be set to an asymmetrical interval [−4, 10] on thebasis of “0”. In this case, 3.90 bits (=log₂(4+1+10)) are consumed toselect the above-mentioned asymmetrical interval.

As can be seen from the above-mentioned examples, the CQI_(Delta)probability distribution is changed according to the Rx scheme, so thatthe range for indicating the CQI_(Delta) value according to theCQI_(Delta) probability distribution must be effectively selected,resulting in reduction of the number of required Tx bits.

The above-mentioned embodiment of the present invention is able to usethe following scheme. If the CQI_(Delta) distribution is symmetrical onthe basis of “0” (e.g., if the MMSE-based receiver is used), it ispreferable that the CQI_(Delta) range be symmetrical on the basis of“0”. Otherwise, if the CQI_(Delta) distribution is asymmetrical on thebasis of “0” (e.g., if the MMSE+SIC receiver is used), it is preferablethat the CQI_(Delta) range be asymmetrical on the basis of “0”.

In the meantime, in fact, the CQI is always quantized so that thequantized CQI is then transmitted. Thus, the transmitters/receivers mustpre-recognize the CQI_(Delta) range. The CQI_(Delta) range must bepre-engaged by the transmitters/receivers.

According to the above-mentioned embodiment, information indicating theRx scheme of the receiver may be transmitted to the transmitter at aperformance-information exchange step of each entity in an initialtransmission beginning process executed between the transmitter and thereceiver.

For example, if the receiver is the user equipment (UE), the userequipment (UE) may inform the Node-B of specific information indicatingwhich one of Rx schemes is used at the UE performance reporting stepcontained in the communication beginning step. If the receiver is theNode-B, the Node-B may inform the user equipment (UE) of the Node-Breceiving (Rx) scheme over a broadcast channel (BCH) and so on.

However, it should be noted that the above-mentioned embodiment is ableto use all the arbitrary methods capable of transmitting the Rx-schemeinformation of the receiver to the transmitter, and a detailed methodfor transmitting the Rx-scheme information to the transmitter is notlimited to the above-mentioned methods and can also be applied to othermethods as necessary. Therefore, the range for indicating differentCQI_(Delta) values according to different Rx schemes of individualreceivers may be differently selected by users.

In the meantime, according to another embodiment, the present inventionprovides a method for establishing the range of a channel-informationdifferential value, irrespective of different Rx schemes of thereceivers, so that the established differential-value range can becommonly used by all receivers. In this case, the probabilitydistribution of the channel-information differential value is changedaccording to the Rx schemes, this embodiment establishes thedifferential-value range of channel information in consideration of allRx schemes capable of being used by the receiver, and a detaileddescription thereof will hereinafter be described.

As described above, the CQI_(Delta) range must be pre-recognized by thetransmitters/receivers, so that the CQI_(Delta) range must bepre-engaged by transmitters/receivers.

However, the CQI_(Delta) range is basically determined by the Rx schemeof the receiver, and the MIMO system is able to use various receivershaving differential Rx schemes. In this case, provided that differentCQI_(Delta) ranges are selected according to individual Rx schemes ofthe receivers, the complexity of the MIMO system may excessivelyincreases.

Therefore, the above-mentioned embodiment uses the same CQI_(Delta)range, irrespective of the CQI_(Delta) distribution caused by adifference between Rx schemes of the receiver, so that the complexity ofthe MIMO system can be decreased.

More specifically, the above-mentioned embodiment provides a method forestablishing the common CQI_(Delta) range in consideration of all theCQI_(Delta) distributions caused by the Rx-scheme difference of thereceivers.

In the case of establishing the common CQI_(Delta) range, theabove-mentioned embodiment enables the CQI_(Delta) distribution in acommon range to be higher than a predetermined probability value in allRx schemes capable of being used by the receiver. Therefore, the presentinvention can indicate the CQI_(Delta) distributions, each of which hasthe predetermined probability value, according to the individual Rxschemes.

Although the CQI_(Delta) distributions caused by various receivers arechanged, the present invention can commonly use the CQI_(Delta) range.In this case, the method for establishing the CQI_(Delta) range willhereinafter be described in detail.

For example, as shown in FIG. 4, the CQI_(Delta) range capable of beingsimultaneously applied to FIGS. 4( a) and 4(b) is [−6, 10]. In thiscase, 4.08 bits (=log₂(17)) are consumed, so that a gain acquired by theused CQI_(Delta) exists.

As can be seen from the above-mentioned example, although the sameCQI_(Delta) range is used irrespective of Rx schemes of individualreceivers, the process for effectively selecting of the CQI_(Delta)range in consideration of all distributions of the CQI_(Delta) valuesdepending on individual Rx schemes is very important to reduce thenumber of necessary Tx bits.

Therefore, this embodiment proposes the following scheme. If there are avariety of CQI_(Delta) distributions, the CQI_(Delta) range involvingvarious CQI_(Delta) distributions is selected. In a variety ofCQI_(Delta) probability distributions, if a first case, in which theCQI_(Delta) probability distribution is symmetrical on the basis of “0”,and a second case, in which the CQI_(Delta) probability distribution isasymmetrical on the basis of “0”, are detected from the above-mentionedCQI_(Delta) probability distributions, the finally-selected CQI_(Delta)range is determined to be asymmetrical on the basis of “0”.

As described above, if the channel information is transmitted using thespatial differential channel information (e.g., spatial CQI_(Delta)),the above-mentioned second embodiment of the present inventiondetermines the CQI_(Delta) range in consideration of the CQI_(Delta)probability distribution, so that it can more correctly indicate thechannel information with less number of bits. In more detail, a firstexample of the second embodiment provides the method for establishingthe CQI_(Delta) range in consideration of differential Rx schemes ofindividual receivers, and a second example of the second embodimentprovides the method for establishing the CQI_(Delta) range inconsideration of all Rx schemes capable of being used by the receivers.

The above-mentioned second embodiment has discussed the method forselecting the range indicating the spatial CQI_(Delta) information. Inthis case, it is assumed that there is no error caused by thequantization interval (or level) of the spatial the CQI_(Delta)information.

However, if the quantization interval of the CQI_(Delta) informationincreases although the same CQI_(Delta) range is used, it is difficultto correctly indicate a variation degree of channel information, whereasthe number of Tx bits of the CQI_(Delta) information is reduced.Otherwise, if the CQI_(Delta) quantization interval is reduced, thevariation degree of channel information can be more correctly indicatedwhereas the number of Tx bits of the CQI_(Delta) information isincreased. Therefore, the following embodiment of the present inventionprovides a method for effectively establishing the quantization intervalto effectively transmit CQI_(Delta), and a detailed description thereofwill hereinafter be described.

The simplest method for establishing the quantization interval may beconsidered to be a method for uniformly dividing the CQI_(Delta) range.However, in the case of considering the CQI_(Delta) probabilitydistribution, the CQI_(Delta) range can be more effectively quantized.In other words, CQI_(Delta) frequently occurs in the high-probabilitypart contained in the CQI_(Delta) probability distribution, so that thisCQI_(Delta) value is more precisely quantized. CQI_(Delta) occasionallyoccurs in the low-probability part contained in the CQI_(Delta)probability distribution, so that it is preferable that this CQI_(Delta)value be quantized to have a quantization interval wider than that ofthe high-probability part.

FIG. 5 is a graph illustrating a method for quantizing a differentialchannel information value according to an embodiment of the presentinvention.

If the CQI_(Delta) probability distribution appears as shown in FIG. 5,a narrow quantization interval is assigned to a specific area (A) havinga probability distribution value higher than a predetermined thresholdvalue (T), and a wide quantization interval is assigned to another area(B) having a probability distribution value higher than thepredetermined threshold value (T). In FIG. 5, the threshold value (T)can be established in various ways in consideration of the number ofbits required for indicating a corresponding CQI and the number ofavailable bits for indicating the number of required bits. Also,although the graph area of FIG. 5 is divided into the area A having thenarrow quantization interval and the other area B having the widequantization interval, the quantization interval of the presentinvention is not limited to the above-mentioned example. If required,the quantization interval may also be established in different waysaccording to distinctive areas classified by two or three thresholdvalues.

In the meantime, if the method for establishing the quantizationinterval according to the above-mentioned embodiment is applied to aspecific case in which various CQI_(Delta) probability distributionsdepending on Rx schemes of the receiver occur, the following operationsare executed.

FIG. 6 is another graph illustrating a method for quantizing adifferential channel information value according to an embodiment of thepresent invention.

As can be seen from FIG. 6, the CQI_(Delta) probability distributionappears in different ways according to Rx schemes of the receiver. Forexample, the CQI_(Delta) probability distribution based on a first Rxscheme is denoted by “CQI_(Delta) 1”, the CQI_(Delta) probabilitydistribution based on a second Rx scheme is denoted by “CQI_(Delta) 2”,and the CQI_(Delta) probability distribution based on a third Rx schemeis denoted by “CQI_(Delta) 3”, as shown in FIG. 6.

In this case, the above-mentioned embodiment considers all the differentCQI_(Delta) probability distributions depending on the Rx schemes of thereceiver, so that it establishes the CQI_(Delta) probability intervalaccording to the considered result. In more detail, the presentinvention considers “CQI_(Delta) T” in which a total of “CQI_(Delta) 1”,“CQI_(Delta) 2”, and “CQI_(Delta) 3” have been considered, assigns anarrow quantization interval to the “A” area having a probabilitydistribution value higher than the predetermined threshold valueaccording to the consideration result of “CQI_(Delta) T”, and assigns awide quantization interval to the “B” area having a probabilitydistribution value less than the predetermined threshold value accordingto the consideration result of “CQI_(Delta) T”.

The above-mentioned description has disclosed the quantization methodfor indicating the differential channel information value “CQI_(Delta)”.If the quantization interval is established as described above, thepresent invention can more correctly indicate channel information withless number of bits.

The above-mentioned CQI reporting scheme description has been related tothe exemplary case for receiving the signal via a single unit frequencyband. Provided that the receiver selects the unit frequency band havingthe best channel status and transmits only a CQI of the selected unitfrequency band, and the transmitter performs a necessary service via theselected unit frequency band at the CQI, only one unit frequency bandrequires the CQI. The above-mentioned case is appropriate for asingle-user environment, but is inappropriate for a multi-userenvironment, so that an effective method is needed.

For the convenience of description and better understanding of thepresent invention, it is assumed that the term “unit frequency band” isa single unit, in which arbitrary frequency bands having similar channelresponses are bound, in a relatively-wide frequency band. It is assumedthat the term “band” or “frequency band” is the above-mentioned unitfrequency band on the condition that there are no comments about othersituations.

In the meantime, the scheduling problem generated when the CQI istransmitted to only one preferable band will hereinafter be described.

If preferable frequency bands of multiple users (i.e., multi-user) aredifferent from each other and their preferable frequency bands are notoverlapped with each other, no problem occurs. In this case, theremaining users other than the selected user are not unable to use thecorresponding frequency. If each user transmits only one preferablefrequency band, unselected users basically lose the opportunity forreceiving desired services. Therefore, in order to solve theabove-mentioned problem, a CQI value of several frequency bands must betransmitted, so that a multi-user diversity gain can be effectivelyacquired.

If the CQI value corresponding to several frequency bands istransmitted, an amount of CQI Tx data increases by a predeterminedamount corresponding to the selected frequency band. For example, ifthree better frequency bands are selected, and their CQIs andfrequency-band indicators are transmitted to the selected frequencybands, the Tx amount of CQI increases three times, and additionalinformation for the indicator of the selected frequency band must betransmitted.

As described above, if the CQI corresponding to several frequency bandsis transmitted, the MIMO communication environment requires much more Txinformation. If the CQI is transmitted in units of each antenna, the Txinformation amount increases by times of the number of Tx antennas. Inthe meantime, if the CQI is transmitted in units of a codeword, the Txinformation amount increases by times of the number of codewords.Therefore, the CQI Tx amount associated with several frequency bandsexcessively increases in the MIMO system than as compared to asingle-antenna system.

Therefore, under the aforementioned situation in which the CQIassociated with several frequency bands must be reported, thisembodiment provides a method for effectively reducing the CQI Tx amount.

Generally, an entire frequency band of a frequency flat fading channelhas the same channel environment, however, the frequency selectivefading channel has different channel characteristics according toindividual frequency bands. Typically, the frequency bandwidth havingthe same channel characteristics is called a coherent bandwidth. Amulti-path channel having many channel paths has a shorter coherentbandwidth in inverse proportion to the number of channel paths.

Therefore, if some bands having a good channel status are selected andCQI values of the selected bands are transmitted, and the above bandsexceed the coherent bandwidth, the similarity among corresponding CQIvalues is decreased. Therefore, the above-mentioned excessive reductionof the selected CQI information is undesirable.

However, if the MIMO system is used so that the CQI for each antenna orcodeword must be transmitted to each selected band, another situation isprovided. In other words, the CQI of each antennas or codeword isdifferently established according to individual frequency bands, but adifference of CQI values of any frequency band is almost similar tothose of the other frequency bands. A detailed description thereof willhereinafter be described on the condition that two codewords arereceived in the MIMO system.

FIG. 7 exemplarily shows a channel-value distribution of each codewordwhen two codewords are received over several unit frequency bands.

As shown in FIG. 7, if the receiver receives two codewords, a CQI of thefirst codeword and a CQI of the second codeword are greatly changed ineach frequency band, but a difference between the two CQIs of the firstand second codewords is not greatly changed although a current frequencyband is changed to another frequency band. The CQI calculation of eachcodeword is affected by channels of other codewords, so that the CQIcalculation processes of individual codewords are affected byinterferences almost similar to each other.

In FIG. 7, a first codeword 1 and a second codeword 2 have different CQIvalues according to individual frequency bands, but differential CQIvalues (e.g., CQI_(Δ1), CQI_(Δ2), CQI_(Δ3)) of individual frequencybands have similar values.

Although the example of FIG. 7 has described the CQI values ofindividual codewords, it should be noted that the above-mentionedprinciple may also be applied to another case in which the CQI for eachantenna is considered. Thus, the following embodiment of the presentinvention will exemplarily disclose a specific case in which the CQI foreach codeword is used. However, it is obvious to those skilled in theart that the above-mentioned embodiment can also be applied to anothercase in which the CQI for each antenna is used as necessary.

In this way, if several codewords are applied to several antennas, a CQIdifference differently occurs in individual frequency bands, and itsdetailed description will hereinafter be described with reference to thefollowing simulation result.

FIG. 8 is a simulation result illustrating a distribution ofdifferential channel information in each unit frequency band when twocodewords are transmitted.

In more detail, if the MIMO system including 4 Tx antennas and 4 Rxantennas transmits two codewords under a TU channel environment at amoving speed of 3 km/h, the bandwidth of 5 MHz is divided into 4frequency bands, and CQI differences between the codewords of individualfrequency bands are shown in FIG. 8. In this case, the divided fourfrequency bands are denoted by “SB1”, “SB2”, “SB3”, and “SB4”,respectively. And, the differential CQI value is represented by theabove Equation 3.

In the simulation of FIG. 8, the CQI is quantized at intervals of 1 dB.The SIC scheme is used as the Rx scheme. Basically, it is assumed thatthe SIC scheme detects a second codeword after detecting a firstcodeword.

As can be seen from FIG. 8, the differential CQI values of individualfrequency bands are similar to each other, so that the individualfrequency bands have differential-CQI distributions similar to eachother. In other words, as shown in FIG. 8, the differential-CQIdistributions in the frequency bands “SB1˜SB4” are similar to eachother. Generally, the differential-CQI distribution is not wide butnarrow.

Based on the above-mentioned result, the present invention provides amethod for reducing an amount of CQI Tx information, and a detaileddescription thereof will hereinafter be described.

Firstly, although a CQI of the first codeword is completely transmittedat each frequency band, a CQI of any codeword from the second codewordcan be transmitted using only the differential CQI value, instead oftransmission of an entire CQI. In this case, the range for indicatingthe differential CQI and the method for quantizing the differential CQIvalue can be established in different ways according to theabove-mentioned embodiments. By the above-mentioned differential CQItransmission scheme, if another CQI from the second CQI is transmitted,an amount of Tx information can be reduced.

Secondly, in order to transmit the differential CQI of a codeword fromthe second codeword, the present invention may transmit onlydifferential CQIs of some frequency bands, instead of transmitting allthe differential CQIs of all the selected frequency bands. For example,the present invention may transmit only a differential CQI associatedwith a single frequency band. For another example, a maximum value,minimum value, and average value of the differential CQI may also betransmitted. For another example, it is preferable that the best channelbe selected to maximize the Tx efficiency, so that another method fortransmitting a differential CQI corresponding to a frequency bandindicating the best channel environment may be considered.

In this way, if the differential CQI is transmitted to only some bands,the remaining frequency bands at which the differential CQI has not beentransmitted may require a differential CQI of any codeword from thesecond codeword according to a scheduling situation. In this case, aspecific differential CQI is selected from among the transmitteddifferential CQIs, and the selected differential CQI is used as adifferential CQI of a corresponding frequency band without any change,or the weighted sum of the transmitted differential CQIs is calculatedso that resultant value may be used as a differential CQI of acorresponding frequency band.

For example, if the only differential CQI of the best frequency bandindicating the best channel environment is transmitted, the transmitteddifferential CQI may be applied to the remaining frequency bands otherthan the best channel without any change. For another example, if anaverage value (or a differential CQI of a specific band closest to theaverage value) of differential CQIs of several frequency bands istransmitted, the above-mentioned average value (or the differential CQIvalue closest to the average value) may also be applied to a frequencyband at which the differential CQI has not been transmitted.

The effects of the CQI transmission methods according to theabove-mentioned embodiments of the present invention will hereinafter bedescribed with reference to the simulation of FIG. 9.

FIG. 9 is a simulation result illustrating a method for comparing aconventional channel information transmission method with an inventivechannel information transmission method.

Basically, the simulation of FIG. 9 has the same environment as that ofFIG. 8. Additional simulation conditions of FIG. 9 are as follows.

A bandwidth of 5 MHz is divided into 4 frequency bands, and two goodfrequency bands are selected from among the four frequency bands (i.e.,Best-2 Scheme). In this case, a CQI of a first codeword at each of thetwo selected frequency bands is transmitted without any loss. However, aCQI of a second codeword can be transmitted in three ways.

Firstly, it is assumed that a first case is that the CQI of the secondcodeword has been completely transmitted at each of two frequency bands.The first case is denoted by “Full CQI” in FIG. 9.

Secondly, it is assumed that a second case is that a differential CQI isapplied to the second codeword so that differential CQIs associated withtwo frequency bands are transmitted respectively. The second case isdenoted by “Delta Separated” in FIG. 9. In this case, the differentialCQI is quantized with 2 bits.

Thirdly, in the case of applying the differential CQI to the secondcodeword, it is assumed that a third case is that only the differentialCQI of a highest-CQI-value band of the two frequency bands istransmitted. The third case is denoted by “Delta Best 1” in FIG. 9. Inthis case, the transmitted differential CQI is applied to the remainingfrequency bands at which the differential CQI has not been transmitted,without any change.

The simulation results of transmission (Tx) efficiencies of theabove-mentioned three cases are shown in FIG. 9. In FIG. 9, a horizontalaxis is “Es/No” (i.e., Signal-to-Noise Ratio (SNR)), and a vertical axisis the transmission efficiency (i.e., a throughput).

In FIG. 9, if “Es/No” is 20 dB, the Full CQI case has a transmissionefficiency of 13.08 Mbps, the Delta Separated case has a transmissionefficiency of 12.54 Mbps, and the Delta Best 1 case has a transmissionefficiency of 12.01 Mbps. Therefore, if the differential CQI is appliedto each of second codewords, a throughput is deteriorated by 4.1% ascompared to an optimum throughput. If only one differential CQIcorresponding to the best channel from among second codewords is used, athroughput is deteriorated by 8.1% as compared to the optimumthroughput. The above-mentioned throughput deterioration is caused byinaccuracy of CQI feedback information. A transmission amount of CQIinformation is greatly reduced, so that the above throughputdeterioration may be considered to be allowable or endurable throughputdeterioration.

The above-mentioned second embodiment of the present invention hasdisclosed the method for additionally reducing CQI information (i.e., Txamount of spatial differential CQI information) when several codewordsare received via several antennas and/or several unit frequency bands.

The first embodiment may be combined with the second embodiment asnecessary.

For example, if the MIMO system transmits channel quality information ofat least two channels, the MIMO system transmits an average value ofchannel quality information of a total frequency band in associationwith one of the two channels according to the second embodiment. And,the MIMO system configures channel quality information of the otherchannel in the form of differential information (i.e., spatialdifferential information), and transmit the differential-formattedchannel quality information.

According to the first embodiment, the present invention may transmitchannel quality information of each channel using an average value oftotal channel quality information and differential information (i.e.,sub-band differential information) of channel quality information ofsub-bands selected by the receiver.

In this case, the present invention transmits the spatial differentialinformation according to the Rx scheme of the receiver, or configuresthe spatial differential information in the form of a specific valuewithin a differential-value range irrelevant to the receiver's Rx schemeso that the configured result is transmitted. If the spatialdifferential information is established irrespective of the receiver'sRx scheme, it is preferable that the differential-value range beasymmetrical on the basis of “0”.

The sub-band differential information inclines to the positive (+) areaaccording to the first embodiment. It is preferable that the sub-banddifferential information be configured in the form of a specific valuewithin the differential-value range composed of only the positive (+)area.

For example, although the above-mentioned embodiments of the presentinvention have been disclosed the CQI generation and transmission methodon the basis of the 3GPP LTE standard, the inventive CQI generationmethod and the user equipment (UE) thereof are not limited to only the3GPP LTE system, and can also be applied to other communication systems(e.g., IEEE 802-based communication systems) which may require afeedback of downlink channel quality information.

The above-mentioned embodiments of the present invention can be appliedto both the downlink and the uplink. If the present invention is appliedto the uplink, the transmitter may be the Node-B, and the receiver maybe an user equipment (UE). The Node-B may be a fixed stationcommunicating with the user equipment (UE), or may also be called a basestation (BS), a Base Transceiver System (BTS), or an Access Point (AP).The user equipment (UE) may be fixed or have mobility. The userequipment (UE) may also be called a terminal, a User Terminal (UT), aSubscriber Station (SS) or a wireless device.

It should be noted that most terminology disclosed in the presentinvention is defined in consideration of functions of the presentinvention, and can be differently determined according to intention ofthose skilled in the art or usual practices. Therefore, it is preferablethat the above-mentioned terminology be understood on the basis of allcontents disclosed in the present invention.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

INDUSTRIAL APPLICABILITY

As apparent from the above description, the channel informationtransmission method according to the present invention indicates channelinformation with less number of bits, and more correctly indicates thechannel status. For example, the present invention can be applied to notonly the 3GPP LTE system but also other communication systems whichrequire a feedback of downlink channel quality information.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1-10. (canceled)
 11. A method for a user equipment (UE) to transmit achannel quality indicator (CQI) to a base station, the methodcomprising: selecting a preferred set of M subbands among a set of Nsubbands, wherein the set of N subbands spans an entire downlink systembandwidth; reporting the positions of the M selected subbands using acombinatorial index selected from among {0, 1, . . . , _(N)C_(M)−1};reporting one CQI value representing wideband CQI index for the set of Nsubbands; and reporting one differential CQI value reflecting thetransmission of the base station only over the M selected subbands amonga predetermined number of differential CQI values, wherein each of thedifferential CQI values is defined based on an offset levelcorresponding to: a CQI index for the M selected subbands—the widebandCQI index, and wherein the differential CQI values represent more of afirst case in which the offset level has a positive value than a secondcase in which the offset level has a negative value.
 12. The method ofclaim 11, wherein, when the UE selects the preferred set of M subbands,the UE reports the positions of subbands using the combinatorial indexrepresenting the M selected subbands.
 13. The method of claim 11,wherein: each of the subbands comprises X frequency domain units, theentire downlink system bandwidth comprises Y frequency domain units, andthe number of subbands spanning the entire downlink system bandwidth (N)corresponds to an integer number not less than Y/X.
 14. The method ofclaim 11, wherein each of the subbands comprises a plurality ofsubcarriers.
 15. The method of claim 11, wherein the differential CQIvalue is represented as a 2-bit value.
 16. The method of claim 11,wherein the positions of the M selected subbands, the CQI valuerepresenting wideband CQI index and the differential CQI value arereported via a Physical Uplink Shared Channel (PUSCH).
 17. A userequipment (UE) for transmitting a channel quality indicator (CQI) to abase station, the UE comprising: a receiver configured to receivesignals from the base station; a processor configured to select apreferred set of M subbands among a set of N subbands, wherein the setof N subbands spans an entire downlink system bandwidth; and atransmitter configured to report the positions of the M selectedsubbands using a combinatorial index selected from among {0, 1, . . . ,_(N)C_(M)−1}, one CQI value representing wideband CQI index for the setof N subbands, and one differential CQI value reflecting thetransmission of the base station only over the M selected subbands amonga predetermined number of differential CQI values, wherein each of thedifferential CQI values is defined based on an offset levelcorresponding to: a CQI index for the M selected subbands—the widebandCQI index, and wherein the differential CQI values represent more of afirst case in which the offset level has a positive value than a secondcase in which the offset level has a negative value.
 18. The UE of claim17, wherein, when the processor selects the preferred set of M subbands,the transmitter reports the positions of subbands using thecombinatorial index representing the M selected subbands.
 19. The UE ofclaim 17, wherein: each of the subbands comprises X frequency domainunits, the entire downlink system bandwidth comprises Y frequency domainunits, and the number of subbands spanning the entire downlink systembandwidth (N) corresponds to an integer number not less than Y/X. 20.The UE of claim 17, wherein each of the subbands comprises a pluralityof subcarriers.
 21. The UE of claim 17, wherein the differential CQIvalue is represented as a 2-bit value.
 22. The method of claim 17,wherein the positions of the M selected subbands, the CQI valuerepresenting wideband CQI index and the differential CQI value arereported via a Physical Uplink Shared Channel (PUSCH).